A. Field of Invention
The present invention relates to new reagents which facilitate the enolboration of aldehydes, ketones, carboxylic acids, anhydrides, thioesters, esters and tertiary amides and more particularly relates to novel dialkylhaloboranes.
B. Prior Art
Enolborinates are highly useful intermediates in organic synthesis. See for example: Evans et al., J. Am. Chem. Soc., 1979, 101, 6120; Evans et al., Ibid., 1981, 103, 3099; Van Horn et al., Tetrahedron Lett., 1979, 24, 2229; Hirama et al., Ibid., 1979, 24, 2225; Reetz et al., Ibid., 1986, 27, 4721; Hooz et al., J. Am. Chem. Soc., 1968, 90, 5936; Hooz et al., Ibid., 1969, 91, 6195; Mukaiyama et al., Chem. Lett., 1976, 559; Inoue et al., Ibid., 1977, 153; Mukaiyama et al., Ibid., 1979, 559; and Inoue et al., Bull. Chem. Soc. Jpn., 1980, 53, 174. As a result, considerable attention has been paid in the past decade to developing simple methodologies for generating suitable organoboron derivatives, R.sub.2 BX, containing a good leaving group in the presence of a suitable tertiary amine.
The previously employed reagents include dialkylboron triflates, R.sub.2 BOTf, (Mukaiyama et al., Chem. Lett., 1976, 559; Inoue et al., Ibid., 1977, 153; Mukaiyama et al., Ibid., 1979, 559; and Inone et al., Bull. Chem. Soc. Jpn., 1980, 53, 174); ethylene chloroboronate (Gennari et al., Tetrahedron Lett., 1984, 25, 2279); ROBCl.sub.2 (Chow et al., Helv. Chim. Acta, 1986, 69, 604); BCl.sub.3 (Chow et al., Helv. Chim. Acta, 1986, 69, 604); and PhBCl.sub.2 (Hamana et al., Chem. Lett., 1984, 1729. However, these reagents are either difficult to prepare in the pure form or only give moderate conversion to the desired enolborinates.
Further, only one organoboron reagent has been reported to date for the generation of enolborinates from esters. See Corey et al., J. Am. Chem. Soc., 1990, 112, 4976; Corey et al., Tetrahedron Lett., 1990, 31, 3715; Corey et al., Ibid., 1991, 32, 2857; and Corey et al., J. Am. Chem. Soc., 1991, 113, 4026. Essentially no organoboron reagent is now available for the enolization of tertiary amides except for di-n-butylboron triflate, n-Bu.sub.2 BOTf, which has been shown to enolize a special class of reactive tertiary amides, the N, N-dialkyl-2,3,3,3-tetrafluoropropanamides. See Kuoboshi et al., Bull. Chem. Soc. Jap. 1990, 63, 1191; and Joshi et al., J. Am. Chem. Soc. 1988, 110, 6246.
I previously reported the enolboration of two ketones, diethyl ketone and propiophenone with dicyclohexylboron chloride and BCl-9-BBN. Preferential conversion into E enolates was obtained with dicyclohexylboron chloride while primarily Z enolates were obtained with BCl-9-BBN, Chx.sub.2 BOTf and BOTf-9-BBN. See H. C. Brown et al, JACS, 1989, 111, 3441-3442.
We have since discovered a novel class of superior enoboration agents and have additionally discovered that dicyclohexylboron chloride is effective for the selective enolboration of carbonyl compounds other than ketones. These advances in the art were recently described in my articles which appeared as H. C. Brown et al., JACS, 1992, 57, 449-504 and H. C. Brown et al., Ibid., 2716-2721.
The present invention fulfills the need for enolboration agents which exhibit better selectivity and reactivity, are readily accessible and are useful in broader classes of organic compounds.